So you're saying if I used
print header, start_html('MyNewPage'); as the title of the post.pl, I'd write <input type="submit" value="Go" onclick="javascript:window.open('','MyNewPage','...');void(0)" /> ?

No. You're confusing the title of an HTML-Document with the name of a frame/window. It's completely irrelevant what post.pl actually prints. Everything concerning windows happens on the clientside. The HTML i wrote already did all the necessary stuff: on submit-button-click a window is created (without content) an named "form_results" and the form has the target "form_results" which causes the browser to put the response of your post.pl into that window, if he is capable of and configured for handling multiple windows.